Piping systems are typically anchored at two or more ends by the process equipment they connect. As the pipe expands due to thermal growth induced by a hot process fluid, it will deform as shown in FIG. 1. If restraints are required to limit dynamic displacements at locations on the pipe which also have thermal movements, a design conflict will exist. This is true in electrical generating plants, for example, where thermal pipe growth may approach 450 mm (18 inches).
For example, if one was to place rigid restraint at Point A, as shown in FIG. 1A, the pipe would not expand as shown in FIG. 1 and may result in an over-stressed condition in the pipe or an unacceptable load on the process equipment. Industry traditionally has resolved this conflict with the use of snubbers. Snubbers allow the pipe to freely expand but momentarily restrain the pipe during a dynamic event. Both mechanical and hydraulic snubbers commonly have been used to restrain piping systems. The problems with snubbers are that they are complex, require maintenance and have a history of failure. These problems have resulted in costly inspection programs in, for example, the nuclear industry. This in turn has prompted many utilities to reduce snubber populations and consider alternatives.
An alternative to snubbers is the use of gapped supports. The disadvantage of this form of dynamic restraint is that it absorbs no energy and imparts very high impact loads on the structure.
Another alternative to snubbers is the use of energy absorbers. Others have proposed varying forms of energy absorbers. One type uses steel plates to absorb energy through plastic deformation of the plates as disclosed in U.S. Pat. No. 4,620,688. A drawback of this device is its low cycle fatigue life. Other proposed types of energy absorbers are shown in U.S. Pat. Nos. 4,955,467 and 4,901,829. Energy is absorbed by friction in these devices. A major drawback of these patented devices are the large mount of variability in the resulting friction force.
In yet another device, the basic element of the restraint is the multi-stranded helical cable trapped between two plates such that energy is absorbed by the deformation of the cable when vibration occurs. This component of the device, known as an "isolator", has been utilized by the military, in satellites, warships, space shuttles, aircraft, off-road vehicle applications, and other areas for many years. The primary use of isolators has been to minimize the transmission of vibration from one component to another. A major producer of isolators is AEROFLEX International, Inc. of Plainview, N.Y., 11805, assignee of U.S. Pat. Nos. 4,783,038 and 4,190,227 related to isolators.
For other related prior art see U.S. Pat. Nos. 2,421,822, 3,204,911, and 4,397,069.
The inventor first considered the use of helical rope isolators as a pipe restraint in 1984. It was thought that the device would be a good seismic restraint which could replace snubbers on piping systems. Conversations with AEROFLEX, a major supplier of isolators, indicated that they did not think it would work well as an isolator due to the relatively low frequency content of the typical building response to an earthquake. The inventor's thoughts, at that time, were not to isolate the pipe from structure in order to minimize the seismic inputs to the pipe, but to use the device to add damping to the piping system and to maintain pipe displacement to acceptable levels during a seismic event. He recognized that while snubbers limited displacements, they added little damping to the system.
In 1984, accordingly, he built a test set-up to determine the response of a pipe supported with and without the isolators. Test results were very favorable, based on the amount of additional damping the isolator provided the system. He presented the results of the experiment in 1986 at the "Symposium on Current Issues Related to Nuclear Power Plant Structures, Equipment and Piping" at North Carolina State University. Those same results were later published in Nuclear Engineering and Design in 1988. This paper is identified as "Vol. 107, North-Holland, Amsterdam, 1988, pp. 201-204".
Based on the encouraging test results, the inventor began to promote the idea of using isolators as a pipe restraint for seismic and hydraulic transient applications. This was done primarily at Commonwealth Edison Company and through public seminars he gave on piping design.
A commercial piping application occurred in 1990 when Commonwealth Edison elected to try isolators on a piping system which was causing a floor slab, to which the pipe was connected, to vibrate at unacceptable levels. The inventor incorporated helical isolators into a design which differs from the invention and allowed the existing pipe support hardware to be utilized. Pipe thermal expansion was not an issue in that case since the pipe was previously supported by rigid supports at the problem locations. It is also noted that in that particular application, the primary purpose of the device was to act as an isolator. The isolator application decoupled the pipe from the floor slab so as to minimize the pipe vibration input to the floor slab. This was a classical application of isolators. The secondary purpose of the device was to minimize the amplitude of the pipe vibration. This installation was a success.
The use of isolators as a pipe restraint has been promoted by the inventor for a number of years, however, the invention includes a new discovery which has not been previously known.
A principal problem in using isolators is that the conflict of design requirements between pipe thermal expansion and a system for restraint of dynamic events has not been solved. While the use of an isolator can provide for some thermal expansion, it is generally limited to small mounts due to the restraining spring force the isolator imparts to the pipe. This provides for limited use of isolators as pipe restraints since direct use of conventional isolators as a pipe restraint results in the same conflict of design requirements between thermal expansion and restraining dynamic displacements. An isolator is a spring. For thermal expansion considerations, a soft spring is desirable so as to not over-stress the pipe or overload the terminating equipment. For dynamic events, however, a stiff spring is desirable since it is desired to limit the magnitude of the dynamic displacements.
An invention which is an energy absorption and pipe displacement limiting device of simple construction, ease of inspection and minimal maintenance was filed with the U.S. Patent Office on Dec. 16, 1991 as U.S. application Ser. No. 07/808,132. An improvement to this patent application was filed with the U.S. Patent Office on Jul. 16, 1992 as patent application Ser. No. 07/915,477. The filed inventions act as a pipe restraint for connection between a pipe subject to movement due both to dynamic loads and to thermal deformations and an adjacent structure. They are adjustable in turnbuckle-like fashion and possesses symmetric stiffness in both tension and compression which resists buckling under load.
U.S. patent application Ser. No. 07/915,477 offers a number of improvements of the invention filed as U.S. application Ser. No. 07/808,132. The improvements provide for a more efficient use of wire rope, a lighter and more compact restraint design, an additional means of providing for bi-linear spring stiffness properties, a means for limiting the load transferred to the structure and various means which provide for ease of manufacture. These improvements are accomplished through alteration of the wire rope bight geometry and structural housing, and the addition of internal stops. The improvements each of these provide are discussed in the following paragraphs:
Wire Rope Bights--The bights of commercially available wire rope isolators used in the original invention are arranged such that they form a stable geometry when loaded in compression as shown in FIG. 2. This is accomplished by arranging the bights in opposing directions as indicated in FIG. 2. This stable arrangement of bights, however, is not required for the current invention to work since the bights are loaded in shear rather than compression and are stabilized by the structural housing of the invention. The arrangement of bights in the fashion depicted in FIG. 2 provide for inefficient use of the wire rope. This is due to the difference in load which the bights carry when the isolator is loaded in shear as shown in FIG. 2A. Some of the bights are more deformed and thus loaded heavier than the others. This inefficiency can be minimized by forming the bights 32 in a near parallel fashion from a continuous wire rope wound in a single direction as shown in FIG. 6. Furthermore, as disclosed in U.S. patent application Ser. No. 07/915,477, the inefficiency can be eliminated through the addition of a third retaining bar, forming each bight 32 from individual lengths of rope and arranging all of the bights in parallel planes as depicted in FIG. 6B. This method of employing individual lengths of rope and three retaining bars is similar to the arch isolator described in U.S. Pat. No. 4,783,038. In either case, the bights 32 are typically trapped between two retaining bars 74 and 76 as shown in FIG. 7. An alternative method of trapping the bights 32 is the use of a rectangular tube 78 filled with epoxy 80 as depicted in FIG. 7A. This latter method of trapping the bights 32 is incorporated in the design of isolators manufactured by ENIDINE of Orchard Park, N.Y. This method of trapping the bights 32 is extended by the invention to the central load carrying member of the restraint. For example, the central member may be composed of a round tube with holes provided for the wire rope to pass through. The central tube is then filled with epoxy to hold the bights in place as shown in FIG. 8. Alternatively, cast metal can be used to fill the void in lieu of epoxy. A third method of trapping the bights is to cast the bars/central rod around the formed bights.
The arrangement of parallel or near parallel bights 32 as depicted in FIG. 6B provides for bi-linear force-deflection properties without precompressing the bights. With a parallel or near parallel bight geometry, the isolator I.sup.1 is more flexible in shear when initially deflected and becomes increasingly stiffer as the deflection increases as shown in FIG. 17. FIG. 17 is the test results from an isolator I.sup.1 which had an arrangement of parallel bights 32 which were not precompressed. Such bi-linear properties provide for a "soft" spring to allow for pipe thermal expansion and "stiff" spring properties to limit pipe dynamic displacements. The range of the "soft" portion of the spring can be further increased by precompressing the bights as previously disclosed in the U.S. patent application Ser. No. 07/808,132.
The efficient arrangement of the bights 32 can also provide a means of reducing the profile of the pipe restraint structural housing. This is desirable since space is sometimes limited in nuclear and process plants, which would utilize these restraints. Alternate restraining devices provide for a compact overall geometry. A means of reducing the profile of the pipe restraints is accomplished by arranging the bights in a fashion depicted in FIG. 12.
Structural Housing--U.S. patent application Ser. No. 07/808,132 utilized a rectangular shaped structural housing for retaining the bights. The arrangement of bights 32 in the fashion disclosed in U.S. patent application Ser. No. 07/915,477 and depicted in FIG. 8 more readily allows the use of a circular tube 150 for the structural housing. This is desirable since tubing of various diameters and thicknesses are readily commercially available. The use of round tubing offers an alternate means of precompressing the bights 32, a means of protecting them from damage and a means of reducing the chance of human injury when the bights are deflected during pipe movements.
A structural housing 150 composed of circular tubing also provides for a means for efficient manufacturing to accommodate varying stiffness and load capacities. By manufacturing bights 32 in standard size groups, the stiffness and load capacity of a restraint 1 may be increased by simply adding more bight groups to a longer rod and structural housing as depicted in FIG. 15. Alternatively, bight groups of three or four may be located around the central rod as depicted in FIGS. 16, and 16A respectively.
Internal Stops--The forces associated with piping hydraulic transients and seismic events are seldom accurately known. Industry designs for such forces using "nominal" design loads. While nominal design loads are chosen which are thought to envelope the maximum actual loads, occasionally the actual loads exceeds the nominal design loads. Pipe restraints are selected based on their capacity to carry a nominal design load. Exceeding this load on the restraint described in U.S. patent application Ser. No. 07/808,132 may result in plastic deformation of the wire rope bights due to excessive deflections. Such excessive deflection may also cause an overstress in the piping system. As a means of limiting such excessive deflections and still provide a safety margin in the force carrying capacity of the restraint, the displacement is limited by internal stops as disclosed in U.S. patent application Ser. No. 07/915,477. The bight retaining bars may act as stops or stops may be welded to, or an integral part of, the central rod. Alternatively, stops may be fastened to the central rod by means of shear pins. Such shear pins are sized such that they are the weak link of the various load carrying members of the restraint. Their size are also dictated by the safety factor desired in the restraint rated load. The use of shear pins offer the additional ability of absorbing a large mount of energy should they be deformed plastically. This provides additional protection to the piping system under extreme dynamic events and limits the load transmitted to the structure to the shear pin-failure load.